Germanium overcoated microbridges

Superconducting microbridges are of great interest for application in SQUID devices. To date, however, these Josephson elements have exhibited certain shortcomings which have limited their utility. Microbridges are extremely sensitive to electrical noise, and can be destroyed by minute currents at room temperature; unprotected microbridges can even be destroyed by electrostatic discharge when handed from one person to another. To ensure their survival, these weak links are generally protected with shunts and/or current-limiting series resistors until cooled to liquid helium temperatures. Further, the temperature range of operation of thin film bridges is limited by self heating effects. And for bridges in certain materials, bridge characteristics can degrade in time through oxidation. It is suggested that all three of these problems can be alleviated by the simple expedient of overcoating with a dense semiconductor such as germanium. This overcoat will protect against oxidation. The semiconducting germanium will shunt the bridge electrically at room temperature, but will behave as a dielectric at liquid helium temperatures and not affect bridge behavior. Finally, the germanium provides additional thermal conductivity and thermal mass at liquid helium temperatures. These initial experiments have verified this conjecture in part for niobium microbridges. (Auth.)